This invention relates to optical lenses, and in particular it relates to an improved method and mold assembly for making plastic lenses.
The principal use for lenses of the present type is as opthalmic lenses for use in eyeglasses, and for this reason the present invention will be described especially with respect to such lenses. However, it should be understood that lenses made in accordance with the present invention also have utility in other fields requiring high quality optical surfaces such as photography, instrument lenses, optical filters and the like.
It has been known to form optical lenses or the like out of a plastic material, and in particular out of a cross-linkable resin monomer that is polymerized and solidified by heat curing while in a mold. Exemplary of such prior processes and mold assemblies are those shown in the U.S. Pats. to Beattie, No. 2,542,386, Weinberg, No. 3,056,166, Slyk, No. 3,136,000, Grandperret, No. 3,222,432, Campbell, No. 3,605,195, Goodwin et al No. 3,821,333, and Beattie, No. 3,931,373. Some of these patents, as well as an earlier U.S. Pat. to Tillyer, No. 2,304,217, have also suggested the desirability of casting a finished lens ready for use (except for being cut around its periphery to fit a particular spectacle frame) rather than casting a semi-finished lens still requiring additional surface treatment such as blocking, generating, grinding, or polishing.
However, these previous mold assemblies and processes have suffered from certain major disadvantages which are overcome by the method and mold assembly of the present invention.
The resin materials used in accordance with these techniques tend to shrink when heated during curing. Depending on the specific material used, such shrinkage can range between approximately 12 to 17 percent. Consequently, in the absence of any corrective action, the liquid will separate from one or both mold surfaces and the finished lens will harden spaced from one or both of the mold surfaces and hence it will not conform to the shape of such mold surfaces. Such imperfections may be acceptable when, as is the case in many of the above cited prior patents, the sole purpose is to form semi-finished lenses to be kept in stock by a lab and subsequently further surface treated to fit a specific prescription. However, lenses cast in this manner would obviously be totally useless if the purpose is to form a finished lens directly from the mold requiring no further surface treatment.
Attempts have been made heretofore to take some corrective action to compensate for shrinkage of the liquid as it is polymerized and solidified. Firstly, it is common practice, as described in many of the above noted U.S. Patents, to provide a gasket around the periphery of the space between the two mold parts of a resilient material which can yield under the spring force urging the mold parts together. An improvement of this technique is described in the U.S. Pat. to Grandperret, No. 3,222,432 and Goodwin et al, No. 3,821,333. As described therein, the gasket is formed of a material having the property of softening at elevated temperatures so as to facilitate movement of the mold parts towards each other at the elevated curing temperatures. However, even this improvement has not satisfactorily solved the problem. The gaskets shown in the Grandperret and Goodwin et al patents are essentially of the "T" shaped variety wherein a ring having a "T" shaped cross-section concurrently holds the two mold parts and forms a seal around the periphery of the cavity between the mold parts. We have found that a disadvantage of this arrangement is that the softening feature takes place throughout the cross-section of the "T" and as this entire cross-section becomes soft and compressed under the spring force holding the mold parts together, it is not practical or possible for that portion of the "T" forming the seal between the mold parts to move with perfect parallelism throughout its entire circumference. Accordingly, while the mold parts might well move together and remain in contact with the liquid, they would tend to move in a non-parallel manner with a resultant disadvantage that the finished lens has unwanted prism characteristics, i.e. it is not of the desired thickness at all points, i.e. it is thinner than desired at some points and thicker than desired at other points. Once again, if the purpose of the molding procedure is to form a finished lens, ready for use without further surface treatments, a lens having undesirable prism properties is totally useless and must be discarded.
The problem of providing a perfect finished lens by having the mold parts move toward each other in a perfectly uniform manner and maintain contact with the liquid throughout the process is complex and difficult enough when forming a perfectly spherical lens. However, the complexity of this problem is compounded when the operator attempts to make a complex lens, i.e. one having further corrections including a cylindrical correction, i.e. the providing of an axis in the finished lens perpendicular to the central axis of the mold assembly and/or providing a multifocal segment. Indeed, we believe that with currently known techniques it is not possible to reliably and consistently form finished plastic lenses having both cylindrical correction and a multifocal segment. In prior techniques the problem has been avoided to some extent by attempting to provide the multifocal segment and/or the cylindrical correction on the concave mold surface (the convex side of the lens) which apparently is less likely to separate from the shrinking and hardening formulation then is the convex mold surface. Such an arrangement is shown for example in the Calkins et al U.S. Pat. No. 3,946,982. However, even this has not been entirely satisfactory, and there has been little or no success in providing such corrections on the concave side of the finished lens.
Hence, there exists a need for improvements in the field of plastic optical lenses which will permit the manufacture of more perfect optical lenses in a manner which will overcome the above described difficulties and disadvantages in the present state of the art.